Epidemiological studies have shown that the _4 allele of apolipoprotein E (apoE) is associated with an increased risk for Alzheimef's disease (AD) while the _2 allele is associated with a decreased risk. ApoE has 3 common isoforms in humans, E2, E3, and E4. Genetic, biochemical, and animal studies strongly suggest that apoE is likely to influence AD pathogenesis via effects on the metabolism of the 38-43 amino acid amyloid-[3 (Al3) peptide. By crossing amyloid precursor protein (APP) transgenic mice that develop AD-like pathology with Apoe / mice and transgenic mice that express human apoE isoforms, we have found that the presence ofapoE is critical in the process by which A[3 converts from soluble to fibrillar forms (amyloid) with neuritic plaque formation and cerebral amyloid angiopathy (CAA). The expression of human apoE (as compared to murine apoE or no apoE) markedly delays Al3 deposition suggesting that apoE also plays an important role in A_ clearance. New data suggest that the LDL receptor (LDLR) should be re- explored as a potential modulator of apoE/A_ clearance in vivo. Determining the effects of altering the levels of different apoE isoforms on At3 in vivo as well as the mechanism(s) underlying these effects is likely to lead to importanl insights into AD and CAA pathogenesis and treatment. Thus, we hypothesize that the level and composition of human apoE isoforms will alter both the timecourse and amount of AI3 deposition in an isoform-specific fashion via effects on AI3 clearance and that human apoE isoforms play a role in CNS A[3 clearance in part via LDLR. These hypotheses will be tested in the following aims: Aim 1: To determine if the level of expression of the human apoE isoforms (via different techniques) influences A]3-related pathology in APP transgenic mice. Aim 2: To assess the mechanism(s) by which apoE influences CNS A[3 metabolism in vivo we will use a novel CNS to blood A[3 effiux assay and a new brain A[3 microdialysis technique. Aim 3: To investigate the role of the apoE receptor, LDLR, in A_3 metabolism and related pathology in vitro and in APP transgenic mice. Aim 4: To determine the role of the cholesterol effiux pump, ABCA 1, in modulating the levels and metabolism of apoE, cholesterol, phospholipid, and A[3 in the brain.